A SIW is known as an alternative interconnect for high-speed and high-frequency signaling. A SIW offers lower transmission losses and excellent immunity to electromagnetic interference (EMI) and crosstalk in comparison with conventional planar transmission lines. Due to its benefits in the high-frequency regime, many SIW-based components have been introduced for microwave and millimeter-wave applications such as antennas, filters, power dividers and phase shifters.
These microwave components are designed to operate within a certain fixed frequency band in microwave and antenna applications. Unfortunately, in many of the available applications tuning is desirable, for example, to provide an antenna array with beam steering capability. For these applications, phase shifters within the antenna array are controllable to create different beam forming networks and result in different radiation patterns. Thus, in prior art designs SIWs are used for signaling only for fixed frequency applications or a separate tunable element is used to provide tunability.
For fixed applications, SIW technology is usable for providing a fixed phase shift. A simple example is a delay-line phase shifter, which gives a phase shift according toφ(f)=β(f)d  (1)
where φ is the total phase shift and β is the phase constant of a SIW. β can be expressed as:
                              β          ⁡                      (            f            )                          =                                                            (                                                      2                    ⁢                    π                    ⁢                                                                  ɛ                        r                                                              ⁢                    f                                    300                                )                            2                        -                                          (                                  π                                      W                    eff                                                  )                            2                                                          (        2        )            
Weff represents the effective SIW width whose properties are equivalent to that of a rectangular waveguide with solid side walls having Weff width. Since β(f) is a strong function of frequency due to the dispersive nature of the waveguide, the phase shift will be varying rapidly over a wide frequency range. This type of phase shift has been implemented. A ferrite-based SIW phase shifter has also been proposed where a ferrite toroid is deposited in an air hole. That said, such a structure has yet to be constructed.
It would be advantageous to provide a SIW that is tunable.